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80 SMT007 MAGAZINE I JULY 2019 the aluminum pads where components need to be assembled. The print thickness depends on the application. Any of the conventional print- ing techniques can be used, including screen, stencil, etc. The aluminum surface does not need any surface cleaning or preparation. Once this treatment compound is printed, it must be thermally cured to leave the pad sur- face treated, active, and ready to accept solder. Cured treatment compound is non-conductive and makes room for easy print registration. To attach a component, it needs solder— via printing or plated bumps—placed on the activated pad and then passed through a reflow oven. The compound overcomes the aluminum oxide layer and allows the forma - tion of a true metal-to-metal bond between the solder and the aluminum on the pads. Thus, both the electrical properties and the bond strength are far superior to silver epox - ies. In addition, it can be stored at room tem- perature and reused multiple times [5] . Figure 3 shows an overview of the process to use the surface treatment. Surface Treatment Cure Temperature: PET Constraints and Solders The surface treatment requires a low-tem- perature cure of less than 100°C. The required dwell time ranges from a few seconds to a min- ute depending on the oven type and air flow. Conveyor-based reflow ovens in PCB assembly houses can cure the printed surface treatment. A variety of solders can be used to attach to aluminum using a surface treatment. However, the type of solders that can be used on Al-PET substrates is limited by the PET film. Table 2 lists key properties common to heat-stabilized PET films. Figure 3: Overview of the process using surface treatment to solder to aluminum. Table 2: General properties of heat-stabilized polyethylene terephthalate film (PET film) [6 & 7] .